1. Field of Invention
This invention relates to the field of backscatter tags and, in particular, to systems for communicating with backscatter tags.
2. Description of Related Art
In RFID systems using modulated backscatter, a reader transmits a radio frequency (RF) signal, and the tags electronically reflect the RF signal. Batteryless or passive tags draw the energy required to run their circuitry from the received RF signal from the reader, while battery assisted or semipassive tags can use a battery to energize their circuitry. For passive tags, where the tag is powered by the signal emitted by the reader, the downlink from the reader to the tag is typically the bottleneck in the link budget, because of the received power threshold required to power up the tag. By using a battery to provide power to the tag's circuitry, semipassive tags relieve this downlink bottleneck, thus producing a significant increase in range. A modulated backscatter system may use either passive or semipassive tags, or a combination thereof.
In either case, in a modulated backscatter based RFID system, the tags electronically reflect the signal received from the reader, while modulating the backscattered signal with data stored in the tags. In addition, the tags may shift the frequency of the reflected signal, in order to separate the modulated backscatter from any unmodulated reflections of the reader signal from other scatterers. Additionally, readers at one stage of a business process may write new data onto tags, and the new data may be read by readers at later stages in the business process.
The downlink from reader to tag is often the bottleneck in RFID systems. Therefore, for passive tags, the received signal must be strong enough to energize the tag. Semipassive tags (or passive tags which can store energy for extended periods) may not use the received signal from the reader to energize the tag receiver circuitry. However, for standard RFID protocols in which tags only perform modulated backscatter when commanded to do so by the reader, the received signal strength must still exceed the threshold required for reliable demodulation of the reader's commands.
There are many situations where it is difficult for the received signal strength to exceed the threshold required for reliable demodulation of the reader's commands in conventional RFID systems. For example, it is difficult in short-range applications where the presence of moisture (e.g., for produce in cold chain applications), liquids (e.g., for pharmaceutical applications) or metal (e.g., in warehouse applications) impair the link. It is also difficult in long-range outdoor applications (e.g., location and tracking of trailers in a yard, or cars in an automobile dealership).
Many existing RFID communication protocols can only support one tag communicating with the reader at a time. Simultaneous transmissions from multiple tags within communication range of a reader typically lead to collisions of the backscattered signals. The collisions must be resolved using collision resolution algorithms or multiple access algorithms whose typical objective is to ensure that tags ultimately transmit one at a time to the reader. The range for RFID systems using passive tags is typically determined by the downlink from reader to tag, which is responsible for energizing the tag.
However, when using passive tags which can store either RF energy or energy gathered from other sources, the downlink signal from the reader may not be the only source for powering the tag during reader-tag communication. In this case, the bottleneck in the communication system may become the uplink. The link budget of the uplink must account for the round-trip propagation loss from the reader to the tag and back. In free space, this loss is proportional to 1/R4, where R denotes the range between the reader and the tag. Similarly, for semipassive tags, the uplink can become the bottleneck, since the downlink link budget only needs to be such that the tag circuitry can detect the reader signal, and the tags do not need to be powered.
Broadly, it is known to transfer power in inductive systems. For example, U.S. Patent Pub. No. 2009/0096413 A1, filed on May 7, 2008 by Partovi, discloses a system for variable power transfer in an inductive charging system. In the Partovi system, multiple receiver/energizer coils are used to recharge tags, mobile phones, MP3 players, radios and other types of portable devices. Additionally, an RFID reader detects the RFID tags in a conventional manner. However, Partovi does not disclose the dual use of an energizer as repeater.
U.S. Patent Pub. No. 2008/0082360 A1, filed on Sep. 27, 2007 by Bailey, discloses an inventory monitoring and control system wherein the weights of items before and after dispensing are measured in order to determine the amount of product inventory that has been dispensed. In the system taught by Bailey, an energizer coil energizes RFID tags which are later read by a reader. However, the Bailey energizer coil does not also operate as a repeater.
U.S. Patent Pub. No. 2007/0224938 A1, filed on Apr. 28, 2006 by Jung, teaches a vehicle control system with a reader/energizer which includes an energizer, a demodulator, and decoder circuits for energizing and reading an RFID tag.
Additionally, US Patent Pub. No. 2007/0120683 A1, filed on Nov. 25, 2005 by Flippen, teaches an implantable electronic medical device. In the implantable device disclosed by Flippen, a transponder in the implantable electronic medical device includes an induction circuit. The induction circuit is energized by a remote hand held reader. Thus, the implantable device taught by Flippen may be both energized and read by a reader/energizer device.
U.S. Pat. No. 4,333,072, issued to Beigel, describes an inductively coupled RFID system in which power to an RFID tag is derived from an alternating magnetic field. The magnetic field originates in a reader/energizer coupled inductively to the tag antenna, and rectified by a rectifier in the antenna. The resulting DC charge stored in a capacitor in the tag.
U.S. Pat. No. 7,551,070, filed on Aug. 31, 2006 by Talty, discloses a wireless sensing system including plural energizers in a predetermined area. The energizers transmit FR energy to tags in order to recharge them. Additionally, an energizer/reader interrogates the tags. U.S. Pat. Nos. 6,172,609, 7,018,361 and 7,348,884, also teach prior art energizer/readers.
However, none of the foregoing references disclose a tag system in which an energizer can also serve as a repeater.
All references cited herein are incorporated herein by reference in their entireties.